Venturi system for producing pressures lower than ambient static pressure typically locate the low pressure tap in the neck or throat of a converging/diverging section configuration. Fluid flow velocity in the neck region is limited to Mach 1 so that the pressure may never go below a level established by the Mach 1 velocity limit.
When the venturi low pressure tap is used to lower the boiling temperature of a coolant liquid, it has been the practice to use low boiling temperature coolants, such as those in the "Freon" family. Freon 11 is commonly used because it has a lower boiling temperature than, for example, water, at a given pressure level. However, the heat of vaporization of Freon, or of other liquids, do not compare favorably with that of water. The basic problem in substitution of water as the coolant is that the boiling temperature is generally higher than a desirable temperature for coolant usage.